Added Text.Pandoc.Data (non-exported) if embed_data_files selected.

This module just exports the association list of embedded data files,
which is used by Shared.
Eliminated use of TH in test suite.
Use hsb2hs preprocessor instead of TH for embed_data_files.

This should work on Windows, unlike the TH solution with
file-embed.
Makefile: Added sdist.